1. Technical Field
Embodiments described herein relate to a wireless field device.
2. Related Art
Conventionally, at places where a cable cannot be wired, for example, in water supply or sewer facilities such as manholes, a flowmeter that has a wireless communication function and serves as a wireless field device is attached to a pipe through which water flows.
In collecting data or inspecting a facility, a worker receives data from a flowmeter in a manhole, for example, by performing a wireless communication with the flowmeter using a hand-held terminal which serves as a wireless communication device.
Field devices include various devices having a communication function such as a pressure gauge, a differential pressure gauge, a thermometer, a level meter, and a flowmeter and devices such as a valve positioner that have a communication function and perform a measurement or control (valve positioner) that directly relates to a process. Field devices other than a valve positioner are called (industrial) transmitters.
Devices which receive a signal from such field devices and perform indication, recording, control, etc., such as an indicator, a recorder, an adjuster, a distributed control system device, an alarm device, etc. are system-side devices and are called receivers as opposed to the above-described concept of transmitters.
Assuming, as mentioned above, the use at places where ordinary wiring cannot be made, wireless field devices are of a battery-driven type. To enable the wireless field devices to operate for a long time, the wireless field devices are provided with a lithium battery as a power source.
FIG. 8 shows the system configuration of an example wireless network which uses such wireless field devices. As shown in FIG. 8, a wireless network topology 1 includes I/O devices 2-6, routing devices 7 and 8, and a gateway 9 and has a star-mesh configuration.
Each of the I/O devices 2-6 is a sensor such as a differential pressure gauge, a pressure gauge, or a thermometer or a manipulation end such as a valve positioner and has a wireless communication function as supported by IEEE 802.15.4.
The routing devices 7 and 8 have an advertisement function of issuing an advertisement to nearby devices on a regular basis and a routing function of sending route information and messages. Alternatively, the I/O devices 2-6 may be provided with the routing function.
The gateway 9 has a function of connecting the wireless network topology 1 and a plant network 10 and thus realizes connection between a control system 11 and the I/O devices 2-6.
In the example of FIG. 8, the I/O devices 2 and 3 can perform a wireless communication with the gateway 9 via the routing device 8 and the I/O devices 4-6 can perform a wireless communication with the gateway 9 via the routing device 7.
An I/O device 12 does not receive advertisements from the routing device 7 or 8 and does not belong to the wireless network topology 1. For example, such a situation occurs immediately after a time point when the I/O device 12 is powered on after power-on of the other I/O devices 2-6.
FIG. 9 is a sequence diagram which allows the I/O device 12 to join the wireless network topology 1. The sequence diagram of FIG. 9 is directed to a case that the I/O device 12 receives an advertisement from the routing device 8 and joins the wireless network topology 1 which complies with, for example, the wireless communication standard ISA 100.11a which utilizes IEEE 802.15.4.
Referring to FIG. 9, at step SQ1, the routing device 8 periodically sends, to nearby devices, an advertisement for urging them to join the wireless network topology 1. The I/O device 12, which has not joined the wireless network topology 1 yet, receives the advertisement sent from the routing device 8.
After receiving the advertisement, at step SQ2 the I/O device 12 sends, to the routing device 8, join requests that are directed to a system manager 91 and a security manager 92 provided in the gateway 9, respectively.
After receiving the join requests, at step SQ3 the routing device 8 transfers, to the system manager 91, the join request that is directed to the security manager 92.
After receiving the join request, at step SQ4 the system manager 91 transfers the join request to the security manager 92. The system manager 91 and the security manager 9 may be provided outside the gateway 9.
At step SQ5, the routing device 8 transfers, to the system manager 91, the join request that is directed to the system manager 91.
After the security manager 92 receives the join request, at steps SQ6-SQ8 a security join permission is finally communicated to the I/O device 12.
Furthermore, when the system manager 91 has received, from the I/O device 12, the join request directed to itself, at steps SQ9 and SQ10 a system manager join permission is finally communicated to the I/O device 12.
At step SQ11, the I/O device 12 sends a security confirm request to the system manager 91.
At step SQ12, the system manager 91 communicates the security confirm request to the security manager 92.
At step SQ13, the security manager 92 sends a security confirm response to the I/O device 12.
After completion of the procedure of FIG. 9, the I/O device 12 can send process data such as a differential pressure, a pressure, a temperature, or the like to the gateway 9 at a constant cycle. The constant cycle is a time interval (e.g., about 1 second to 1 hour) that can be set arbitrarily by the user.
Process data that has been transmitted to the gateway 9 is processed by the control system 11, whereby monitoring by the user is performed or a process control is performed by sending a wireless signal via the gateway 9 to an I/O device that is an operation terminal such as a control valve.
Each of the I/O devices 2-6 and 12, after joining in the wireless network topology 1, usually suspends the operation of the internal circuits to save energy consumption of the built-in battery. Each of the I/O devices 2-6 and 12 is activated at the constant cycle, that is, only when it is necessary to send process data. After sending of process data, the operation of each of the I/O devices 2-6 and 12 is suspended until the next calculation and sending of process data.
FIG. 10 is a graph showing an example current consumption characteristic of the I/O device 12 before and after joining in the wireless network topology 1. The horizontal axis represents time and the vertical axis represents the current consumption. The sequence process of FIG. 9 is executed in a period Ta that is from power-on of the I/O device 12 to completion of joining (indicated by a broken line). In the period Ta, the I/O device 12 is always kept operational to receive signals successively and hence the current consumption always has a large value Ia.
In a period Tb that starts from the completion of joining, the I/O device 12 is activated at a constant cycle T, that is, only when it is necessary to send process data. The I/O device 12 repeats a cycle of calculating and sending process data and then being kept inactive until the next calculation and sending of process data. In a period T1 of each cycle T, the I/O device 12 is kept inactive and the current consumption has a small value Ib (<Ia). In the other period T2, the I/O device 12 is operational to send process data. Each arrow denoted by symbol tp indicates that the I/O device 12 is sending process data to the gateway 9 by a wireless communication.
As can be seen from FIG. 10, after the completion of joining, the combination of the suspension period T1 in which the current consumption is Ib and the active period T2 in which the current consumption is Ia is repeated at the constant cycle T, and the suspension period T1 accounts for a large part of the cycle T. Therefore, the energy consumption of the battery can be reduced by elongating the constant cycle T.
JP-A-2003-134030 discloses a technique for adding a wireless communication function to a field device, and JP-A-2003-134261 discloses a communication system which uses field devices having a wireless communication function.
Incidentally, in constructing the wireless network shown in FIG. 8, the gateway 9 needs to be installed and activated first. However, because of delayed purchase of equipment, configuration of the plant network 10 or the control system 11, or some other reason, the gateway 9 may be installed after installation of the I/O devices 2-6 and 12 or the routing devices 7 and 8.
On the other hand, in general, field devices used as the I/O devices 2-6 and 12 and the routing devices 7 and 8 do not have a power-on/off switch outside the cabinet. This is because such field devices used in a dangerous atmosphere are required to have a pressure-resistant, explosion-proof structure. In general, such field devices are powered on/off by attachment/detachment of a battery.
Although it is conceivable to provide a switch inside the cabinet of a field device, opening the lid of the cabinet to turn on or off the switch is a burden to the user. It is not permitted to open the cabinet at a place where pressure resistance and explosion proof are required. Therefore, it is necessary to turn on the power switch or insert a battery immediately before installing a field device at an intended site.
As a result, in a state that the gateway 9 has not been installed yet, field devices used as the I/O devices 2-6 and 12 and the routing devices 7 and 8 have to wait for an advertisement (step SQ1 in FIG. 9) in the power-on state for a long time. That is, the state that the period Ta in which the current consumption has the large value Ia (see FIG. 10) continues until the gateway 9 is installed and activated.
In actual measurement examples, the current consumption in the period Ta is several tens of times the average current consumption in the period Tb (see FIG. 10) and one-day delay of installation of the gateway 9 results in consumption of energy that would be consumed in several months if the gateway 9 were installed. In battery-driven wireless field devices, the battery life is an important issue and the fact that the battery energy may be consumed in the state of the period Ta shown in FIG. 10 for a long time is a serious problem.
The same situation occurs when the gateway 9 has failed: A large amount of battery energy is consumed in field devices used as the I/O devices 2-6 and 12 and the routing devices 7 and 8 until completion of replacement of the gateway 9.
Powering on field devices after completion of installation or replacement of the gateway 9 is not practical for the following reasons. First, as mentioned above, the cabinet cannot be opened at a place where pressure resistance and explosion proof are required.
In a large plant, several hundreds or several thousands of field devices may be distributed. Powering on all the field devices after completion of installation or replacement of the gateway 9 not only increases personnel expenses but also increases, for example, the probability of occurrence of trouble due to scuffing of the lid of a cabinet and danger in a pressure-resistant area due to incomplete closure of a lid.
Even if a battery is inserted into place and the lid of its cabinet is closed in a safe area such as a user work bench area, the battery energy is consumed uselessly until a start of regular communications.